I like spooks.
They make the car look 10x better and probably let me drive the car 10 mph faster than otherwise. I like that spook design, did you get at Canby?
I agree on the color, too. Body color spooks are weird on street cars. Sometimes the historic 510s can get away with it.

Personally I'm more drawn to Original gauges. That opinion would change if you don't have a stock dash, though, like Demo243's post. Then having aftermarket gauges would be a GREAT idea.
I know you mentioned the mechanics of the gauge being your primary concern, but if you want to keep the old gauges and just wanted to actually see them at night, LED lights make things a helluvalot brighter.

Everything that everyone else is talking about ^^^
Since it hasn't been mentioned just yet, I am here to advocate for POR-15 for less major rust problems or things you don't feel comfortable taking a welding torch to (like floor pans, transmission tunnels, and parts of the undercarriage).
Use it for places that have a bit of surface rust and aren't structurally unstable. Take the metal down as close to bare metal as possible then lather POR-15 on (careful: it stinks). You can paint over it later.

As mike said, impossible to troubleshoot with a corroded and junky box.
If the fuse box is shit, then it could be any of a hundred problems coming from there. I would dunk the box in vinegar, clean up any sketchy contacts, then take a volt meter to your alternator. If car is stock, you'll still have a voltage regulator and can trouble shoot with that.
A "fix" that I've seen from many people is to cut out the volt reg. and put in an internally regulated alternator. It's not as much work as you think, but might not be the issue you have.
When you connect your battery, check to see if you get a spark on the negative connector (with the pos. still plugged in). If yes, then something in your system is constantly hot, and would explain why the car died.
I think that finding the real problem and soldering here and there would be easier than buying a new harness.

This could be out there, but what is the condition of all the connections? As in, are the grounds clean, connectors corroded, or any loose / sloppy connections?
I was pulling 12.01 volts out of my brand new alternator and relatively new battery. I cleaned my contacts and vinegar dunked my fuse block and it jumps to 13.95 volts.

That makes sense.
I'm not particularly worried about it filling up, based on the amount of junk coming out of either the block or valve cover. Plus, I have a plug on the bottom I can drain from if really needed.
I'll be on the lookout for a larger filter to increase air flow. Should I be worried about driving it long distance as is?

So I sense that the debate between oil filter and dump line on oil catch cans is a relatively subjective discussion. I have elected to go with a filter for aesthetic reasons. My question for you all is, what size filter would you recommend / are you running?
As i understand it, the filter's main use is a pressure release, as the pipe coming out of the crankcase generates a little pressure. Given that, is a filter this size enough?

Yeah something I've noticed is that having a datsun invites ALL KINDS of conversation. Sometimes with people that... just... don't get it. i.e. Fart Canners.
Hell yeah! It helps when you have one of the most classic 510 looks out there! There's something about the fat tire on the 13" libres. I absolutely love the color, too. Reminds me of someone I know who needs to get THEIR orange 510 back on the road...

I'm confused by what you mean by bumps through the shifter. Is it moving from road bumps without you touching it? Is it a vibration, or an erratic shaking? Vertical bump? or side to side? Do you hear a noise of the transmission smacking the underside of the car? Any noise to go along with it? If you have your hand on it, don't push down into the transmission, because you'll definitely feel bumps and vibration. It seems normal to me that you'd feel a bounce, while holding onto your shifter, while going over bumps. I could be wrong.
Maybe it would be different if it was a small bump and you got A LOT of movement.
In my mind the best thing would be to look at the transmission mount and how the shifter moves while standing still. If the trans. crossmember allows for movement, it's not doing its job.
That happens after a visit to my local taco truck.

I just had a good one a few days ago. A guy pulled up next to me at a light in a mid-70's f150 and the passenger (i think the driver's brother?) started talking about his old Datsun 620. Said it was his "mini-ranch-truck". Light turned green, both sped to the next red, continued the conversation. This happened three lights in a row. Kinda funny, him talking literally down about 5 five feet from his window to mine.

Don't know about trucks, but for 510's the popular option I've seen (mainly for mikunis and racecars) is a custom-made aluminum sheet. Can be a bitch to manufacture, but in the end, the only measurements that will work are those pertaining to YOUR engine bay. Measure measure measure.